For the production of a melt-processable fluororesin by emulsion polymerization, ammonium perfluorooctanoate (hereinafter referred to as APFO) is widely used as an emulsifier.
However, with APFO, its bioaccumulation potential has been pointed out, and there has been a concern expressed about environmental problems with respect to disposal of APFO. A substitute emulsifier for APFO is therefore demanded.
Therefore, a method for producing the melt-processable fluororesin by using various emulsifiers other than APFO has been proposed, wherein a fluorinated monomer alone or a fluorinated monomer and another monomer are copolymerized in an aqueous medium (for example, Patent Documents 1 and 2).
With the emulsion-polymerization method in Patent Document 1, its applicability for the production of a melt-processable fluororesin was not necessarily high, because the molecular weight of an obtainable fluororesin tended to be low, or the fluororesin had coloration.
In an Example in Patent Document 2, use of CF3CF2OCF2CF2OCF2COONH4 as a polymerization emulsifier for producing polytetrafluoroethylene modified by a trace amount of hexafluoropropylene, is disclosed, but an application for a melt-processable fluororesin is not described.
In an Example in Patent Document 3, CF3CF2OCF(CF3)CF2OCF(CF3)COONH4 as a polymerization emulsifier is disclosed, but it was found that the emulsifier had a higher bioaccumulation potential than APFO.
In Examples in Patent Document 4, F(CF2)5OCF(CF3)COONH4, etc. are disclosed.
Patent Document 1: JP-A-2002-308913
Patent Document 2: JP-B-39-24263
Patent Document 3: JP-A-2003-119204
Patent Document 4: JP-A-2002-317003